As is well known in the state of the art, some metals or metal alloys, such as titanium, zirconium, hafnium, tantalum, niobium, or alloys thereof, are used to form relatively strong links with bone tissue. In particular, metal implants of titanium and its alloys have been known since approximately 1950 for their properties of binding well to bone tissue. This binding was called osseointegration by Branemark et al. (Branemark et al., “Osseointegrated implants in the treatment of the edentulous jaw. Experience from a 10-year period”, Scand. J. Plast. Reconstr. Surg., II, suppl 16 (1977)).
Although the binding between this metal and bone tissue is relatively strong, it is desirable to improve this binding. There are many methods developed in the state of the art to treat such metal implants to obtain a suitable surface on them to improve their osseointegration. The term “surface” is understood to refer to the superficial layer or most external zone of an implant, composed mainly of the oxide of the corresponding metal, the physical properties of which are clearly different from the massive material that the implant is made of.
Some of these methods are directed to altering the morphology of this superficial layer, increasing its roughness, in order to provide a higher area of contact, and therefore of binding, between the implant and the bone tissue, resulting in higher mechanical retention and strength, that is, in a better osseointegration of the implant.
The reason behind these procedures for increasing surface roughness are the studies carried out in the last few years (Buser et al., “Influence of surface characteristics on bone integration of titanium implants. A histomorphometric study in miniature pigs”, J Biom Mater Res, (1991), 25:889-902; Wennerberg et al., “Torque and histomorphometric evaluation of c.p. titanium screws blasted with 25- and 75-um-sized particles of Al2O3”, J Biom Mater Res, (1996); 30:251-260; Buser et al., “Removal torque value of titanium implants in the maxilla of miniature pigs”, J Oral Maxillofac Implants (1998) 13:611-619; and Lazzara et al., “Bone response to dual acid-etched and machined titanium implant surfaces”, Bone Engineering, chap. 34 (2000) J. E. Davies eds.), which demonstrate that osseointegration of the implant in the short and medium term is improved by a micrometric surface roughness.
Also, other studies (Buser et al. 1991, supra; Cochran et al., “Attachment and growth of periodontal cells on smooth and rough titanium”, Int. J Oral Maxillofac Implants (1994) 9:289-297; Martin et al., “Effect of titanium surface roughness on proliferation, differentiation, and protein synthesis of human osteoblast-like cells (MG63)”, J Biom Mat Res (1995) 29:389-401; Lazzara et al. 2000, supra; and Orsini et al., “Surface analysis of machined vs sandblasted and acid-etched titanium implants”, J. Oral Maxillofac Implants (2000) 15:779-784) have demonstrated that the existence of a superficial layer on the implant of a micrometric roughness improves osteoblast cellular expression, giving rise to better cellular differentiation and better osteoblast expression. The consequence of this effect is an improved osseointegration and a more bone formation.
Also, some more research-based manufacturers, such as Nobel Biocare, have designed such surface treatments so that they increase the thickness and the crystallinity of the titanium oxide layer, as some studies seem to suggest a relationship between the degree of crystallinity and better osseointegration of the implant (Sul et al., “Oxidized implants and their influence on the bone response”, J Mater Sci: Mater in Medicine (2002); 12:1025-1031).
The methods used in the state of the art to increase surface roughness of an implant are very diverse. Among them can be highlighted the application of a coating over the surface, blasting of the surface with particles and chemical attack of the surface.
The common methods of coating the metal implant surface consist in applying a metal coating, normally of titanium, or a ceramic layer, normally of hydroxyapatite, by various known techniques such as plasma pulverisation or plasma spray (Palka, V. et al., “The effect of biological environment on the surface of titanium and plasma-sprayed layer of hydroxylapatite”. Journal of Materials Science: Materials in Medicine (1998) 9, 369-373).
In the case of blasting the surface, particles of various materials and sizes are used, which are blasted on the surface of the implant in such as way as to alter its morphology. Usually, particles of corundum (alumina) are used (Buser et al. 1991, supra; Wennerberg et al. 1996; supra), or particles of titanium oxide (Gotfredsen, K. et al., “Anchorage of TiO2-blasted, HA-coated, and machined implants: an experimental study with rabbits”. J Biomed Mater Res (1995) 29, 1223-1231).
On the other hand, chemical attack of the surface is carried out using various mineral acids such as hydrofluoric acid, hydrochloric acid, sulfuric acid, etc. So, for example, in a series of United States patents by Implant Innovations Inc. (U.S. Pat. No. 5,603,338; U.S. Pat. No. 5,876,453; U.S. Pat. No. 5,863,201 and U.S. Pat. No. 6,652,765) a two-stage acid treatment is described that is used to obtain the commercial Osseotite® surface. In the first stage, aqueous hydrofluoric acid is used to remove the natural oxide layer on the metal surface; in the second stage a mixture of hydrochloric acid and sulfuric acid is used to obtain a micrometric rough surface. In the European patent application EP 1477 141, also from Implant Innovations Inc., a variation of this method is described in which a mixture of hydrofluoric acid and hydrochloric acid is used in the second stage to treat implant surfaces based on titanium and Ti6Al4V alloys.
The combined use of both techniques has also been described, that is, blasting of the implant surface followed by chemical attack. So, Buser (Buser et al. 1991, Buser at al. 1998, supra) described, among other methods, blasting with medium grain alumina followed by etching with a mixture of hydrofluoric and nitric acids; also blasting with coarse alumina followed by chemical treatment with a mixture of hydrochloric and sulfuric acids. Similarly, Cochran (Cochran et al. 1994, supra) used blasting with fine or coarse corundum particles followed by a chemical treatment with hydrochloric and sulfuric acids to treat a titanium surface. Similarly, Choi Seok et al. (KR 2003007840) described blasting with calcium phosphate particles followed by treatment with a mixture of hydrochloric and sulfuric acids. Equally, in the document WO 2004/008983 by Astra Tech, a method of treatment of implant surfaces was described that combined blasting with fine and coarse particles of titanium oxide followed by treatment with hydrofluoric acid. Also, Franchi (Franchi et al., (2004) “Early detachment of titanium particles from various different surfaces of endosseous dental implants”, Biomaterials 25, 2239-2246) and Guizzardi (Guizzardi et al., (2004) “Different titanium surface treatment influences human mandibular osteoblast response”, J Periodontol 75, 273-282) described blasting with fine and coarse zirconia particles followed by an unspecified acid treatment.
Regarding thermal treatment, Browne (Browne et al. (1996), “Characterization of titanium alloy implant surfaces with improved dissolution resistance”, Journal of Materials Science: Materials in Medicine 7, 323-329) and Lee (Lee et al. (1998), “Surface characteristics of Ti6Al4V alloy: effect of materials, passivation and autoclaving”, Journal of Materials Science Materials in Medicine 9, 439-448) described the treatment of a previously untreated titanium alloy with hot air at 400° C. for 45 minutes to achieve better resistance to dissolution and a higher thickness of the oxide layer; although the thickness achieved was only 4 nm.
By means of these methods, therefore, surfaces with micrometric roughness are obtained but with a very much reduced surface titanium oxide thickness, which entail the disadvantages of not having a very stable titanium oxide layer and not reducing the release of metal ions to the medium.
The state of the art, therefore, continues to require alternative methods of treating the superficial layer of metal implants that provide a micrometric surface roughness and with improved chemical composition and thickness in order to optimise the process of their osseointegration.
Spanish patent application 200701518 of the present authors describes a method for obtaining a surface of titanium-based metal implants which is virtually free from impurities, with a thickness which is approximately three times the thickness of conventional surfaces, and with a micrometer-scale roughness and morphology (FIGS. 1a and 2a) optimizing the osseointegration and bone anchorage processes.
Said method consists of projecting particles of zirconium oxide under pressure to sandblast the external area of the implants, followed by a subsequent chemical treatment with a particular combination of acids and by a final thermal treatment. The use of a mixture of sulfuric acid and hydrofluoric acid, as well as the combination of these three treatment and the conditions of the final thermal treatment had not been described until then.
The present inventors have now found that in the sandblasting process of the previous method, the substitution of the type of particles together with the modification of their size and the pressure at which they are projected allows obtaining an alternative surface of a metal implant with a micrometer-scale morphology which is different but also suitable for optimizing the osseointegration and bone anchorage processes.
Therefore, the method of the present invention allows obtaining alternative surfaces of titanium-based metal implants with optimized properties in relation to chemical composition, thickness, and micrometer-scale roughness and morphology which translate into good osseointegration and cell response properties.